Project Summary This fellowship will support my postdoctoral training in the Lawrence lab, which has recently shown that the human XIST gene can be targeted to chromosome 21 to silence the extra chromosome in Down Syndrome stem cells. Down Syndrome, caused by trisomy for chromosome 21, is a highly common and important disorder in itself, but it is now recognized that trisomy 21 is additionally a cause of early onset Alzheimer Disease. Essentially all individuals with trisomy 21 develop amyloid plaques at an early age, and ~80% develop Alzheimer-type dementia by the age of 60-65. The APP gene, the precursor of amyloid ? peptides, is on Chr21 and recognition of this relationship contributed to identifying the link between amyloid and Alzheimer Disease. However, the biology and full role of APP is not well understood, and it is not clear whether there are other factors on chromosome 21 that aggravate or modify the effects of trisomy for APP. This proposal will utilize and expand an innovative experimental stem cell system to investigate the extent to which APP or other chromosome 21 genes increase the accumulation of amyloid ? peptides and Tau protein in Down Syndrome cells. We will also seek progress in developing the potential of the XIST RNA to silence gene expression locally and chromosome wide. XIST RNA normally silences one X chromosome in females, providing a natural mechanism of dosage compensation that we are repurposing towards the goal of chromosomal therapy for Down Syndrome patients. In addition to using this system to better understand the causes of AD cell pathology, we will also seek to evolve the potential for XIST chromosomal therapy by two approaches. The first approach, will evaluate if XIST silencing of chromosome 21 in Down Syndrome neuronal cells can overcome Alzheimer Disease pathology. The second approach will combine the necessary and minimal XIST RNA domains that can silence a human chromosome 21 or a region like the APP locus, to create a molecule small enough to facilitate delivery of this RNA to patients. Hence, this project seeks understanding of Alzheimer Disease pathology in trisomy 21 and also seeks to advance technology towards chromosomal therapy.